The present invention is directed to devices and methods for generating light with electrodeless high intensity discharge (HID) lamps. More particularly, the present invention provides high intensity discharge lamps driven by a radio-frequency source without the use of electrodes inside a gas-filled vessel (bulb) and related methods. Merely by way of example, such electrodeless HID lamps can be applied to applications such as parking lots, street lights, warehouses, stadiums, security, ports and harbors, large and small buildings, vehicle headlamps, billboard lighting, building facade lighting, airports, bridges, agriculture and horticulture lighting, architectural lighting, stage and entertainment lighting, medical illumination, microscopes, projectors and displays, ultraviolet (UV) water treatment, UV curing, any combination of these, and the like.
High intensity discharge lamps provide extremely bright and broad spectrum light source. Typical conventional electroded HID manufactured today contains a bulb with a mixture of gas and metal halides that are excited to form a plasma using a high current passed through closely-spaced electrodes. This arrangement, however, suffers from deterioration of the electrodes over time, and therefore a bulb with continual degradation of performance and limited lifetime.
Electrodeless high intensity discharge lamps driven by radio frequency (RF) sources have been proposed in the prior art. Some configurations include a metal halide fill encased either in a bulb or a sealed recess within a dielectric body forming a waveguide, with RF energy being provided by an external source such as a magnetron or solid-state RF driver and introduced into the waveguide and heating the plasma resistively. Another example is provided by U.S. Pat. No. 6,737,809 B2, which shows a different arrangement. This patent shows an electrodeless bulb and a dielectric cavity forming a part of a resonant RF circuit with an RF driver (which produces and amplifies electromagnetic energy at radio frequencies) to provide excitation. Several limitations, however, exist with this approach. The dielectric cavity is spatially positioned around a periphery of the electrodeless bulb in an integrated configuration, which physically blocks a substantial portion of the light emitted from the bulb. In addition, the integrated ceramic and quartz bulb configuration is difficult to manufacture and limits the operation and reliability of the plasma-enclosing bulb. Furthermore, the dielectric material used in this approach is often costly because of the difficultly in manufacturing and also due to the high-purity that is required to achieve competitive performance for most lighting applications.
In another approach disclosed in U.S. Pat. Nos. 8,283,866 and 8,294,368, an air-cavity resonator with grounded coupling elements is used to provide advantages over previous dielectric waveguide/resonator approaches. The air cavity resonator eliminates the need to use costly dielectric material and the bulb is not surrounded by a dielectric material resulting in more efficient operation of the lamp. However, this approach has its own limitations. The air-cavity resonator has two coupling elements. An input coupling element that is connected at one end to the RF source and at the other end to the body of the resonator which is at ground potential. The output coupling element that is connected to the bulb at one end and at the other end is connected to the resonator body. An air gap separates the input and output coupling elements. The input coupling element couples the RF energy from the RF source to the output coupling element which in turn couples the RF energy to the bulb, ionizing the gas in the bulb, and vaporizing the metal halide to emit light. The bulb which is made from quartz or a transparent/translucent ceramic operates at a high temperature. In case of a quartz bulb for example the surface temperature of the quartz envelope can exceed 800° C. It is critical for efficient as well as reliable operation of the bulb to maintain the temperature of the bulb within a certain range. The output coupling element in addition to coupling RF energy to the bulb serves the critical role of removing enough heat from the bulb to ensure reliable operation of the bulb but not too much heat that results in excessive cooling of the bulb and less efficient operation. Consequently, the output coupling element has to be designed properly to provide the necessary operating temperature range for the bulb. Furthermore, depending on the metal halide used in the bulb and the slurry/epoxy used to attach the bulb to the output coupling element, the design of the output coupling element has to be changed to achieve the desired operating temperature for the bulb. However, changing the design (such as dimensions or material composition) of the output coupling element to accommodate the necessary temperature profile for the bulb, changes the operating frequency of the resonator. It is highly desirable to use the same lamp/resonator body with different types of bulbs and different output coupling element designs without having to change other dimensions of the resonator to ensure the operating frequency of the lamp stays constant.
From above, it is seen that techniques for improved electrodeless high intensity discharge lighting are highly desired.